Goals: Explorer 1 was the first spacecraft successfully launched by the U.S. It orbited Earth every 115 minutes, 358 kilometers (220 miles) above the surface at its closest point and 2,550 kilometers (1,580 miles) at its farthest. It carried instruments to measure cosmic rays, micrometeorites, and its own temperature, transmitting this data until May 23, 1958.

Accomplishments: Explorer 1 found a radiation belt around Earth, which came to be called the Van Allen radiation belt after James Van Allen, the scientist who built the cosmic ray detector. It also proved the United States could successfully launch a satellite into Earth orbit.